Bangladesh security agencies have secretly detained scores of opposition activists many of whom have later been killed, an international rights group said Thursday.

Amid heightened political tensions, the Human Rights Watch report was released only days after a high profile government critic briefly disappeared.

HRW said Bangladeshi authorities have detained hundreds of people in secret locations since 2013, including at least 90 last year.

The New York-based group said it has documented 21 cases of detainees who were later killed, and nine others whose whereabouts are unknown.

"Bangladesh security forces appear to have a free hand in detaining people, deciding on their guilt or innocence, and determining their punishment, including whether they have the right to be alive," Brad Adams, HRW's Asia director said.

"The disappearances are well-documented and reported, yet the government persists in this abhorrent practice with no regard for the rule of law," he said.

Among those missing is Sajedul Islam Suman, 37, a Dhaka neighbourhood chief of the main opposition Bangladesh Nationalist Party (BNP).

Sumon's sister Sanjida Islam said Rapid Action Battalion (RAB) officers detained her brother and five other BNP activists on December 4, 2013. This was one month before a controversial 2014 general election which the BNP boycotted.

"Some 20 construction workers who were on the site told us RAB officers picked them up and led them away on a RAB van. They were never returned," Islam told AFP.

"For the last three years and eight months we've knocked on doors, gone to every agency's office and met the home minister to know my brother's whereabouts," she said.

The RAB and police have denied involvement in the disappearance.

Political tensions are growing again as a national election is widely expected next year. And mothers of 22 of the missing activists have set up a group, Mayer's Daak (Mother's Call), to seek government answers on the cases.

Islam said the family was hopeful her brother, a father of two, would be found, just as police found a prominent government critic this week just 18 hours after he was allegedly abducted.

Authorities said Farhad Mazhar, a poet, writer and dissident, was found on a coach around 200 kilometres (120 miles) from the Dhaka, where he lives.

The 69-year-old was escorted back to the capital and a kidnapping case was filed.

"He was blindfolded, picked up on a vehicle and taken away," Abdul Baten, joint commissioner of detective police, quoted Mazhar as saying.

According to HRW, in the first five months of 2017, 48 disappearances were reported. Local rights groups said at least 326 people have disappeared since 2009, mostly opposition activists.

Officials of the BNP and its ally Jamaat-e-Islami, the largest Islamist party, have said tens of thousands of their activists and supporters have been arrested by the government in recent years.-AFP